Web service to optimally schedule a set of tasks over time. Used by WizWeek.
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README.md

Task schedule optimizer web service

This is a web service written to optimally schedule a set of work tasks given the following:

  • A list of tasks to accomplish each with a business value ("reward") and estimated time to complete in hours. Tasks may optionally also have a deadline and a minimum start date.
  • Weekly work time blocks
  • Appointments when project work cannot be scheduled
  • Time zone

Request sample and format explanation

Here's a sample post which will inform the explanation of each piece of it:

POST /

{
  "timeZone": "America/New_York",
  "weeklyTaskBlocks": [
    [],
    [{"start": "10:00", "end": "12:00"}],
    [{"start": "9:00", "end": "10:00"}, {"start": "11:30", "end": "14:30"}],
    [],
    [],
    [{"start": "16:00", "end": "18:00"}],
    []
  ],
  "appointments": [
      {"title": "Meeting", "start": "2016-04-17T16:00:00Z", "end": "2016-04-17T18:00:00Z"}
  ],
  "tasks": [
    {"title": "Newsletter", "estimatedHours": 3, "reward": 9, "deadline": "2016-04-20T22:00:00Z", "startOnOrAfter": "2016-02-17T15:00:00Z"},
    {"title": "Reimbursement", "estimatedHours": 2, "reward": 15, "startOnOrAfter": "2015-02-18T15:00:00Z"}
  ],
  "startTaskSchedule": "2015-02-16T14:00:00Z",
  "endTaskSchedule": "2015-02-28T22:00:00Z"
}

The timeZone should be the name of the time zone of the weekly time work blocks, e.g. "America/New_York".

The weeklyTasksBlocks is an array of 7 arrays. Each of the 7 lists correspond to the generally available project work blocks in a given week. List 0 corresponds to Sunday, list 1 to Monday, etc. So in the example above, that represents having weekly project work availability on Monday from 10am-12pm, and then on Tuesday from 9am-10am and 11:30am-2:30pm, and Friday from 4pm-6pm. Note that the times are given in a 24 hour clock format. This might correspond to weekly normal working hours, for instance.

Next appointments is a list of time periods that project work cannot be scheduled even if that appointment falls during one of the normal weekly time blocks that could be used for project work.

Then comes the tasks list. Each task should have a title field which describes the task, then estimatedHours and reward (a measure of the business value of the task). Currently estimatedHours must be a whole number of hours. A task may optionally also have deadline, startOnOrAfter, or both of those fields. A task represents a project you want to accomplish during those weekly project work hours.

Finally, the startTaskSchedule and endTaskSchedule give the start and end times for the calculation to take place over. Often startTaskSchedule will be the current time and endTaskSchedule should be far enough into the future to allow for scheduling all the tasks assuming there are no conflicts with deadlines.

Here's the response our sample request would give:

[
  {
    "title": "Newsletter",
    "start": "2015-02-16T15:00:00Z",
    "end": "2015-02-16T17:00:00Z",
    "finish": false
  },
  {
    "title": "Newsletter",
    "start": "2015-02-17T14:00:00Z",
    "end": "2015-02-17T15:00:00Z",
    "finish": true
  },
  {
    "title": "Reimbursement",
    "start": "2015-02-20T21:00:00Z",
    "end": "2015-02-20T23:00:00Z",
    "finish": true
  }
]

What it gives is an optimal arrangement of the into chunks of time (minimal size of 1 hour) that will fit into the weekly time blocks and work around the specified appointments. They could then be auto-added to a calendar easily. There is one additional field finish for each task that indicates whether that is the final (finishing) work block for that particular task or whether there will still be more work blocks on it to come.

If a deadline cannot be met the service will respond with: {"err":"Could not solve linear program"}

How the optimization works

Here's a general overview of how the scheduler works.

First it has an initial step where it considers all the weekly time blocks and the appointments and maps them to a flat list of "available work hours" and when those available work hours would be. It also translates the deadlines and minimum start dates into indexes into that "available work hours" list.

Next it forms a linear program as follows:

  • Each task will have a set of variables that correspond to all of the different hours that task could be scheduled on. I.e. there are available work hours * number of tasks total variables of the form task[task_index]_hour[hour_index]. If variable task[0]_hour[0] is 1 then that means that you will do task 0 at hour 0.
  • Each variable has a constraint that it must be at minimum 0 and maximum 1.
  • All the task variables for a particular hour have the constraint that they must sum to at most 1 - i.e. you can't schedule more than one hour of tasks on a given hour.
  • The sum of all the hours of a given task must be less than or equal to the estimated hours for that task, i.e. you can't keep doing a valuable task over and over after it's done but can only capture its business value once.
  • The deadline for a given task is modeled as a constraint that the sum of all the scheduled hours for a given task before the deadline be the total estimated time of the task.
  • Likewise, a minimum start time is a constraint that the total hours of the task before the start time be zero.

The objective function of the linear program is the sum of all the reward/hour for each task multiplied by all the hours that task is scheduled.

Because this just uses a linear program with continuous variables, how does this avoid fractional assignments of the variables? Well, assuming the reward/hour of each task is different, and the tasks have whole numbers of estimated hours, then the tasks will naturally pack into full hour slots to maximize the objective function. Because ties in the reward/hour of different tasks is possible, the program adds a small random "nudge" value to that hourly reward to pre-emptively break the ties. The random nudging is done with a fixed seed so it is consistent for a given input.

Internally this uses the golp library which wraps the LPSolvelinear programming solver.

How to use it

Currently in my personal use of this for my own schedule I use Google sheets with a Google apps script as the front-end for this service, which also takes care of fetching appointments from my Google calendar and saving the task blocks to a separate one.

I have started on a web front end for it but haven't gotten very far with it.

Feel free to use it to build your own personal task scheduling system as well!

Deployment

This has been set up to be easily deployed to Heroku as the lpsolve55.so file is bundled with the code and there is an included Heroku Procfile which specifies how to run the Go service.

License and Acknowledgements

This idea of optimizing your tasks is based on an Excel spreadsheet my dad, John Raffensperger users to manging his tasks. For his explanation of it and a link to his spreadsheet, see john.raffensperger.org/.

The bundled lpsolve55.so file is LGPL licensed.

The Go code in this project is MIT licensed as follows:

MIT License (MIT)

Copyright (c) 2015 David Raffensperger

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.